There are a variety of industrial processes that require a pressurized vapor. Pressurized vapor is often produced by pumping a cryogenic liquid and then vaporizing the pumped liquid. Typical cryogenic liquids are those obtained from the fractionation of air into its separate components, namely, liquid nitrogen and liquid oxygen.
Oil and gas drilling applications are key examples of industrial processes that involve the use of pressurized vapor and in particular, pressurized nitrogen. For instance, in enhanced oil recovery, oil fields are pressurized with nitrogen. Nitrogen is also used in gas lift operations where it is injected down hole to decrease the density of oil and help to drive the oil to the surface. Additionally, pressurized nitrogen is often used in various oil and gas well completion operation such as in fracturing, acidizing and cementing.
In oil and gas well drilling and completion applications, a pressurized nitrogen stream is obtained, on site, by pumping liquid nitrogen to pressure and then vaporizing the liquid nitrogen. The pump is driven by an internal combustion engine. The heat required for the vaporization is generated directly from combustion taking place within the internal combustion engine and from shaft work performed by the engine on hydraulic fluid that can be converted into heat energy.
An example of such a device is described in U.S. Pat. No. 4,738,115. The pumping and vaporization system shown in this patent utilizes a diesel engine coupled by a transmission to a dynamometer, a variable displacement hydraulic pump, a fixed displacement lube oil pump and a fixed displacement hydraulic pump. The variable displacement hydraulic pump forces hydraulic fluid to a variable displacement hydraulic motor that in turn drives the cryopump used in pumping liquid nitrogen. The lube oil pump circulates lubrication oil that is used in the various items of equipment and the fixed displacement hydraulic pump drives a circulation pump to circulate a coolant throughout a coolant circuit. The coolant is introduced into heat exchanger to vaporize the liquid nitrogen. Although there is a minor amount of heat that is transferred from the lube oil pump and the fixed displacement pump to the coolant, the majority of the heat required for the vaporization of the liquid nitrogen is generated by a dynamometer. Increasing the load on the dynamometer increases both engine heat output and also, heat to the coolant.
Another example of a nitrogen pumping and vaporization system is disclosed in U.S. Pat. No. 4,438,729. This patent utilizes two internal combustion engines. The first of the internal combustion engine drives a nitrogen pump through a transmission to pressurize the nitrogen. A transmission retarder varies the load on such engine and generates heat that is dissipated into a cooling fluid. The second internal combustion engine drives three hydraulic pumps against adjustable back pressure valves to provide a variable load on the second engine. The heat generated is also transferred to the cooling fluid. Liquid nitrogen is vaporized in a heat exchanger from exhaust gases from the internal combustion engine. It is then subsequently superheated by the heated coolant.
U.S. Pat. No. 4,197,712 discloses a system in which a single engine drives a variable displacement pump to pump oil to a hydraulic transmission that is coupled to a cryogenic pump. Another pump is driven by the same engine to pump oil through a vaporizer and heat exchanger that is used to vaporize the liquid nitrogen. The flow circuit used in driving the hydraulic transmission is selectively back pressured by an adjustable back pressure valve to control the amount of heat generated.
The prior art as disclosed in the above referenced patents presents rather complex systems that have a rather limited range of control. For instance, in U.S. Pat. No. 4,738,115, the amount of heat generated can only be controlled through the loading on the dynamometer. Similarly, in U.S. Pat. No. 4,197,712, the amount of heat generated is controlled by adjustment of the single adjustable back pressure valve employed in the hydraulic circuit used to drive the pump. While U.S. Pat. No. 4,438,729 employs both a adjustable back pressure valve and a transmission retarder to provide two points of adjustment and therefore, the potential of greater heat control, it is difficult, if not impossible, to accurately adjust a transmission retarder and therefore, the amount of heat generated in the hydraulic circuit used in powering the cryopump. Furthermore, the use of such equipment as dynamometers and transmission retarders present unwanted expense and complexity.
A further deficiency in pressurization systems of the prior art is that such systems it is not easily scalable to provide apparatus that is efficient for a particular operation. For instance, in any of the patents discussed above if a greater flow rate of nitrogen is required for a particular application, engine size must be increased to handle the increased pumping and vaporization duty.
As will be discussed, the present invention provides a system for pressurizing and vaporizing a cryogenic liquid that has a greater degree of control than prior art devices and is amenable to be scaled up or down without simply increasing or decreasing the size of the internal combustion engine utilized in such system. Moreover, the system of the present invention is less complex and uses less expensive components than prior art systems.